Black band disease: Elucidating origins and disease mechanisms

Coral diseases were unknown in the scientific community fifty years ago. Since the discovery of a coral disease in 1965, there has been an exponential increase in the number of known coral diseases, as the abundance, prevalence, distribution, and number of host species affected has also significantly increased. Coral diseases are recognized as contributing significantly to the dramatic losses of coral cover on a global basis, particularly in the Caribbean. The apparent sudden emergence of coral diseases suggests that they may be a symptom of an overall trend associated with changing environmental conditions. However, not much evidence has been gathered to address this question. The following studies were designed to build a comprehensive argument to support this hypothesis for one important coral disease—black band disease (BBD). A meta-analysis of clone libraries identifying the microbial communities associated with BBD reveal important information including that a single cyanobacterial operational taxonomic unit (OTU) was by far the most prevalent OTU in diseased samples, and that the alphaproteobacteria, which include some of the most common bacteria in marine waters, were the most diversely represented. The analysis also showed that samples exhibited regional similarities. An fine and ultrastructural characterization of the disease revealed that the cyanobacteria are prolific borers through the coral skeleton, and that the cyanobacteria penetrate coral tissue, leading to their presence ahead of the main migrating disease band. It was further found that apparently healthy corals exposed to toxins found in BBD, exhibited similar tissue degradation to those infected with BBD. Comparing the disease progression to biofilm formation, it was determined that scouting cyanobacteria may contribute to the migration of the disease through progressive biofilm development over intact coral tissue. Together, these studies provide significant evidence for the hypothesis that BBD is an opportunistic disease, caused by common environmental bacteria, facilitated by the changing environmental conditions associated with climate change.